CiviMail: click-through tracking breaks when contact/email deleted

The click-through tracking code is too fragile; the click-through URLs stop working when the addressee is deleted from the CiviCRM database, and it also stops working when the mailing in question is deleted.

This behaviour leaves non-working links in the users' archives, while it should be doable to simply redirect the user properly in such cases (without registering the click-through).